“Or ‘hyper-sitting syndrome’. Energy expenditure is as much of a contribution as calorific intake.”

People’s lifestyles have changed radically in the past 40 years. Gyms might have proliferated but day-to-day exercise has plummeted.

People sit at work all day. Walking is down, online shopping is up, so we either need less food or more exercise.

Sir Muir suggests small changes. A biscuit less a day and doing a bit of walking.

“For every half hour, try 20 minutes sitting, eight standing, and two walking to the printer – that’s a good balance.”

People are being urged to walk more (Image: PA)

Joan Costa-Font, from the London School of Economics worries about the sedentary lifestyle.

“Typically, life in the 21st century might mean a commute into a desk-based occupation and three or four meals a day, leading to many people consuming more calories than their lifestyles require,” he said.

“People no longer have to visit each other to hold a face-to-face conversation when they can simply Skype.

"We jump in the car or on the bus or we use the Tube rather than walking”.

But could the data on falling calorie intake and rising obesity be wrong?

We spend more time sitting down at work (Image: Getty)

“We know very clearly that dietary surveys tend to under report by a surprisingly large amount,” said Susan Jebb, professor of diet and population health at Oxford University .

“There is a degree of optimistic bias, where we like to imagine we ate a bit healthier than we did.”

And underreporting is more common in overweight people.

As Professor Jebb says, “The fact is most of us are overweight. Most people are less active than would be good for their health. And most people are eating more than they need.

“If you spend too much time agonising over which is to blame, you miss the fact that you should just tackle both.

“I’m much more interested in how to get out of this mess than how we got into it.”